Automatic assembly of semiconductor devices

ABSTRACT

The leads of a stem are first formed or bent to accurately position them relative to a pedestal of the stem, the pedestal being that portion of the stem on which a semiconductor pellet is to be mounted. The formed leads are thereafter used as a means for accurately locating the stem at a parts assembling work station at which a pellet is disposed on the pedestal in preselected positional relation with the leads and contacts are disposed on the leads and pressed into engagement with the pellet.

This invention relates to the fabrication of semiconductor devices, andparticularly to "automatic" or machine assembly of mount assemblies ofsemiconductor devices.

One known type of semiconductor device comprises an envelope including amount assembly comprising a flat header having a number of leadsextending therethrough, a semiconductor pellet mounted on the header,and sheet metal electrical contacts extending between and bonded to theleads and various surface portions of the pellet.

Heretofore, the steps of disposing the pellet on the header and mountingthe contacts on the leads and in contact with the pellet were doneeither entirely by hand or with the use of simple jigs, an example ofthe latter being shown in U.S. Pat. No. 3,390,450, issued July 2, 1968,and assigned to the RCA Corporation.

An object of this invention is to provide means for automating ormachine performing these pellet and contact assembly steps.

FIG. 1 is a view, in perspective, of a semiconductor mount of the typemade according to this invention.

FIG. 2 is a view, in perspective, of a jig used in the practice of theinstant invention.

FIG. 3 is a view, in perspective, of a portion of an operating stationof an apparatus, one set of operating tools at this station being shownin retracted position.

FIG. 4 is a view similar to that of FIG. 3 but showing the one set ofoperating tools in advanced, operating position.

FIG. 5 is a view, in perspective and partly broken-away, of anotheroperating station of the apparatus.

FIG. 6 is a cross-sectional view, on an enlarged scale, along line 6--6of FIG. 5.

FIG. 7 is a view of an assembly of parts used according to the instantinvention to assemble the mount shown in FIG. 1.

FIG. 8 is a side view of the assembly shown in FIG. 7 after portions ofthe assembly are bent at right angles to other portions thereof.

FIG. 9 is a plan view of the assembly shown in FIG. 8.

FIG. 10 is a view in perspective showing the mounting of portions of theassembly shown in FIGS. 8 and 9 on stem workpieces to complete the mountshown in FIG. 1.

With reference to FIG. 1, a transistor mount 10 is shown comprising astem 12 including a flat header 14 having two openings 15 therethroughand two leads 16 hermetically sealed through the header 14 by means ofglass seals 18, the leads 16 preferably having rounded upper ends 17.Integral with the header 14 is a pedestal 20 serving as a heat sink fora semiconductor pellet 24 mounted thereon. Two contacts 26 extendbetween each lead 16 and a different surface portion of the pellet 24.One end of each of the contacts 26 has an aperture 32 therethrough andan extending flexed tongue 34 extending into the aperture 32 locking thecontact onto a lead. The other end of each of the contacts 26 has adependent tip 38 bonded to the pellet surface.

Although not shown, a completed transistor is provided by hermeticallyenclosing the mount 10 in a known type enclosure or can.

The mount parts described are bonded to one another by means of solderedjoints. To this end, prior to assembly of the mount, the bottom andselected portions of the top surface of the pellet 24 are provided witha coating of solder, and the contacts 26 are solder clad, as by knownmeans.

The mount 10 is generally of known type and materials, and the variousparts thereof can be made according to known processes. In comparisonwith known mount assemblies, however, one structural modification ispreferably made in the stem 12 to more readily accommodate it to theapparatus used in the herein described embodiment of the invention.While facilitating practice of the invention, this modification is notessential.

The stem modification comprises providing the stem pedestal 20, which isnormally four-sided, with at least two additional corner sides 40 whichare each disposed diagonally between the pedestal sides 42 and 44, thesides 40 being accurately shaped and dimensioned with respect to a flatside 46 of the pedestal at the opposite end thereof.

In the practice of the invention, the starting workpiece is the stem 12,the fabrication of such stems being known. A number of the stems 12,e.g., seven, are first loaded, by hand, on a jig 50. As shown in FIG. 2,the jig 50 comprises a plate 52 mounted on a pair of legs 54, the plate52 having a number of openings 56 each of which is designed to receive astem 12 partially therethrough. Disposed about each opening 56 are fourtabs 58 struck out of the material of the plate 52. The dimensions ofthe jig 50 are such that the leads 16 of a stem 12 pass freely througheach opening 56, while the stem header 14 rests on the jig plate 52 withthe sides of the header loosely confined between the tabs 58, the jig 50thus allowing a substantial amount of lateral movement of the stemsthereon.

After loading of the jig 50, a first process comprises reshaping theleads 16, where necessary, for accurately positioning the leads of eachstem with respect to the pedestal 20 thereof. This step is necessarybecause a lead-to-pedestal positioning tolerance is required in thepractice of this invention which is tighter than is generally achievablewith known stem fabricating processes.

To achieve this lead positioning, the loaded jig 50 is disposed at anapparatus including a plurality of identical stations corresponding innumber to the number of stems on the jig, all the stems thus beingsimultaneously operated on by the apparatus. FIGS. 3 and 4 show aportion of one of the stations of the lead positioning apparatus, suchportion including a pair of jaws 60 and 62 (FIG. 3) for accuratelypositioning a stem 12 relative to the station, and a pair of jaws 64 and66 for reshaping, as necessary, the leads 16 of each stem.

It is noted that the art of positioning one or more workpieces at aworkpiece operating station and for actuating tools towards and intoengagement with the workpieces is quite old and well developed. Becauseof this, various details of the various apparatus used in the practiceof this invention, including means for supporting the jig 50 inrelatively accurate relationship with the apparatus, and means foradvancing and retracting various tools of the apparatus are notdescribed, the provision of such means being well within the ability ofpersons skilled in this art.

Returning to the lead shaping process, the function of the jaws 60 and62 (FIG. 3) is to engage the sides 40 and 46, respectively, of the stempedestal 20 for accurately positioning the stem 12 at the lead shapingstation. As shown, the leading edge of the jaw 62 is flat for engagementwith the flat side 46 of the pedestal 20, while the leading edge of thejaw 60 contains a V-notch 68 for engagement with the diagonal sides 40of the pedestal. In operation, the two jaws 60 and 62 are advanced intocontact with sides of the stem pedestal 20, the jaw 62 providing a stopfor positioning the stem 12 in one lateral direction, and the sides ofthe V-shaped notch 68 positioning the stem in the other lateraldirection. Owing to the loose mounting of the stem 12 on the jig 50 (notshown in FIG. 3) the stem is freely positionable by the jaws.

To prevent jamming of the apparatus, and to accommodate pedestaldimensional variations from stem to stem, one of the jaws is advanced bymeans of a force applied through a force limiting means, e.g., acompressive spring or an air cylinder piston (not shown).

Having accurately positioned the stem, the jaws 64 and 66, which up tonow have been in retracted position, are actuated. As shown in FIG. 3,the leading edge 70 of the jaw 64 contains a pair of V-shaped notches 72and the jaw 64 has a thickness equal to about 2/3 of the length of theleads above the header 14. The other jaw 66, of substantially the samethickness as the jaw 64, is provided with a pair of V-shaped fingers 74,the leading edges thereof including a circular bottomed groove 78, andthe fingers 76 being shaped and dimensioned to mate with the V-shapednotches 72 in the jaw 64.

In operation, the two jaws 64 and 66 are simultaneously advanced, theleads 16 first (generally) being engaged within the wide mouth notches72 of the jaw 64 and moved towards the apex of each notch, and the leads16 then being firmly pressed into the notch apices by the fingers 74 ofthe jaw 66.

The leads 16 are actually hammered between the two jaws 64 and 66, thematerial of the leads thus being overstressed to mold them to the shapeand location established by the mating jaws 64 and 66. As a result ofthis process, the leads 16 are accurately positioned relative to thestem pedestal 20, the upper portions of the leads 16 also being disposedin vertical alignment relative to the top surface of the pedestal 20 tofacilitate, as described below, the mounting of the contacts 26 on theleads.

While a particular means has been described for shaping and positioningthe leads, it is noted that stem lead shaping is a known art, and otherknown means can be used.

The jig 50 is then disposed at an apparatus a function of which is toprovide means for guiding a pellet 24 into accurate position on the stempedestal 20 and for maintaining the pellet in such position until thecontacts 26 are disposed on the workpiece.

A portion of this apparatus, including two stem operating stations isshown in FIG. 5, each station including a pair of stem lifting rods 80(only one pair shown) mounted on a vertically moveable plate 81, a firstmovable plate 82 having a pair of generally V-shaped lead positioningnotches 84 disposed on either side of a third V-shaped notch 86; asecond movable plate 88 having a notch 90 for engaging the pedestal 20,and a third movable plate 92 having a pellet guiding V-shaped notch 94in its leading edge. As shown, the plates 82 and 92 are coplanar, andthe plate 88 is disposed beneath the plate 92.

In operation, with all the plates in retracted position, the jig 50 isdisposed at this apparatus in such position to dispose the stem leads 16of each jig in general alignment with the notches 84 in the first plate82. This plate is then advanced to engage the stem leads 16 within theplate notches 84. The second movable plate 88 is then advanced (by aforce applied through, e.g., a compressive spring) into contact with thesides 40 of the stem pedestal to firmly seat the leads 16 at the apicesof the two notches 84 to thus accurately position the stem pedestal 20relative to the apparatus. The third movable plate 92 is then advancedso that the V-notch 94 therein cooperates with the V-notch 86 in theplate 82 to form a guide or chute 96 (FIG. 6). The walls of theV-notches 86 and 94 are beveled so that the chute 96 has beveled innerwalls. The rods 80 (FIG. 5) are then raised through the jig openings 26(the jig 50 not being shown in FIG. 5) to engage and lift the stemheader 14 to thus position the stem pedestal 20 closely adjacent to(FIG. 6) the undersides of the plates 82 and 92 and directly beneath thechute 96.

A pellet 24 (FIG. 6) is then disposed, either by hand or, preferably, bya known type of vacuum pick-up tool, within the open end of the chute 96and thus in preselected position on the stem pedestal 20, the chute 96maintaining the pellet 24 in position after the pellet disposing meansis removed.

The last step in the process is to dispose a contact 26 on each lead 16and in contact with the pellet 24, the contacts 26 serving as flexedsprings clamping the pellet against the pedestal until the pellet issoldered to the pedestal 20 and contacts 26 in a later operation.

The contact disposing process is preferably done automatically, and tothis end, a plurality of pairs of contacts 26, one pair for each stem 12on the jig 50, are provided connected to an elongated strip 100 (FIG. 7)of a contact assembly 102. The spacing between each pair of contacts 26along the strip 100 corresponds to the spacing between the various stems12 on the jig 50, and the dimensions and relative angles of the contacts26 of each contact pair correspond to the positional relationshipbetween the leads 16 and the pellet 24 as determined by the variouspositioning plates 82, 88 and 92 as shown in FIG. 5. Also, each contact26 is provided with its dependent tip 38.

The contact strip assembly 102 is most simply and inexpensively madeusing known sheet metal stamping and forming procedures. A problemassociated with the use of such procedures, however, is that owing tothe stretching of the sheet metal during the metal-cutting operation, arandom amount of curvature is introduced into the strip assembly 102.This is shown (exaggerated) in FIG. 7.

The problem caused by this random curvature is that it is not possible,without more, to simultaneously align the apertures 32 of all thecontacts 26 on the assembly 102 with all the leads 16 of the variousstems 12 on the jig 50, all the leads 16, as shown in FIGS. 2, 5, and10, being disposed along a straight line.

This problem is solved by bending each of the contacts 26 at rightangles to the strip 100, as shown in FIGS. 8 and 9. While this has noeffect upon the curvature of the material (see FIG. 8), the contacts 26still lying along a curved surface, it does dispose all the contactapertures 32 along a straight line, as shown in FIG. 9.

The elongated strip 100 of the contact assembly 102 is then disposedbetween the jaws of an elongated holder 104, as shown in FIG. 10, andthe holder 104 is moved towards the stems 12 (still disposed at theapparatus shown in FIG. 5, details of this apparatus not being shown inFIG. 10) to first align the contact apertures 32 of each contact 26 witha different lead 16 of each stem 12, and to then push each contact ontoand downward along each lead 16 until the contact tip 38 engages,through the open end of the chute 96 (FIG. 6), the pellet 24 incontact-flexed relationship. The holder 104 then releases the contactassembly strip 102 and is withdrawn.

By virtue of the extending tongues 32 within the contact apertures 30,the contacts 26 remain mechanically locked to the leads 16, while theflexed contacts clamp the pellets 24 in place against the pedestal 20.

While, owing to the curvature of the surface in which the variouscontacts lie (FIG. 8), there is some variation in the height of thecontacts 26 on the leads 16 and thus some variations in the amount offlexure of the contacts 26, these height variations are actually quiteminor (the amount of curvature of the strip 100 being shown quiteexaggerated in FIGS. 7 and 8) and have no adverse effects on theclamping of the pellets 24 against the stem pedestals 20.

The various movable plates of the apparatus shown in FIG. 5 are thenretracted, and the jig 50 is then removed from the apparatus and theelongated strip 100 separated from the various amounts. This latter stepis readily accomplished by providing the strip 100 with perforationswhere the contacts 26 join the strip, whereby a few bending twists aresufficient to break-off the strip.

What is claimed is:
 1. A method of assembling a mount assembly includinga stem comprising a header member and at least two leads extendingtherethrough, and a semiconductor pellet mounted on said header, saidmethod comprising:accurately positioning the header of a stem at a firstapparatus work station and engaging said leads by forming tools at saidstation for shaping said leads to dispose an end portion of each thereofin preselected positional relationship with said header, thereafterdisposing said header in loosely mounted disposition adjacent to asecond apparatus work station and engaging said leads by positioningtools at said second work station for accurately positioning both saidleads and said header relative to said station, disposing, at saidstation, an open-ended guide chute adjacent and leading to said header,and disposing a semiconductor pellet within said chute and on saidheader, said chute providing accurate positioning of said pellet on saidheader.
 2. The method of claim 1 including the subsequent step ofengaging one end of each of two elongated contacts with each of saidleads respectively and the other ends of said contacts with said pelletthrough an end of said chute.